1. Field of the Invention
This invention relates to a touch panel disposed on a display front surface of a display device such as a liquid crystal display device, a display device and a method of producing a touch panel.
2. Description of the Related Art
(A) Transparent conductive films formed on opposing surfaces of upper and lower substrates of a touch panel are corroded by moisture and have low durability. Therefore, it has been attempted in the past to change the material of the upper and lower substrates from a resin, through which moisture easily permeates, to glass through which moisture does not easily permeate, to improve the durability (Japanese Unexamined Patent Publication (Kokai) No. 10-133817).
However, touch panels for use in a stable domestic environment can satisfy the moisture resistance requirement even with the conventional construction described above but when they are used for a long time in a high-temperature high-moisture environment such as inside an automobile, the conventional construction cannot satisfy the moisture resistance requirement.
It has been found for the first time that, as a result of the observations by the inventor in experiments, the moisture enters a space between a pair of glass substrates opposing each other in a touch panel generally available on the market (a touch panel adopted in a commercial liquid crystal monitor for car navigation) in the course of 400 hours when the touch panel is left standing in a high-temperature high-moisture environment of 65° C. and 95% RH, and the touch panel does not function normally after.
When the thickness of a seal portion (in a sectional direction) that bonds the pair of glass substrates, and is possibly one of the route of invasion of the moisture into the commercial touch panel used for the experiments, was measured, it was found to be 20 μm.
On the basis of the new finding, by the inventor, that invasion of the moisture into the touch panel depends on the thickness of the seal portion, this invention aims at providing a touch panel that can be used for a long time even in a high-temperature high-moisture environment by setting a thickness of a seal portion to a predetermined range so as to avoid invasion of the moisture into a space between a pair of glass substrates by permeating through the seal portion, a display device and a method of producing the touch panel.
(B) As a display device of this kind, a display device is known that arranges a transparent touch switch on a display light outgoing side of a liquid crystal display device of a car navigation apparatus, for example.
While watching switches displayed on the liquid crystal display device, an operator directly touches and operates the surface of the touch panel and has the feeling that the operator is operating the switch displayed on the liquid crystal display device. When such a touch panel is employed, switches that have been arranged in the past at the periphery of the liquid crystal display device can be eliminated, and the operation of the liquid crystal display device becomes easier.
In the construction of the touch panel, however, there are four positions at which the touch panel is in contact with air. That is, they are the outer surfaces of a pair of transparent insulating substrates and the outer surface of transparent conductive films formed on the inner side of the substrates. Reflection of incident light from outside always occurs on these outer surfaces in touch with air, and display of the display device disposed at the back of the touch panel becomes more difficult to see.
When the display device and the touch panel are combined with each other, external light is reflected on the surface of the touch panel, and display light from the display and reflected external light are superposed on each other. It has thus been found that external light reaching the display light outgoing side of the display device is reflected by a polarization plate arranged on the display light outgoing side and makes display light more difficult to watch due to reflected external light in the same way as described above in addition to the problem that display light is difficult to watch for the observer due to the influences of reflected external light.
In a touch panel capable of suppressing reflected light from the touch panel and a display device comprising the combination of this touch panel with a display device, this invention aims at providing a touch panel for such a display device, and a display device capable of suppressing reflected light from the display device and improving the display quality of the display device.
(C) One of the touch panels according to the prior art is described in Japanese Unexamined Patent Publication (Kokai) No. 10-133817 described above.
An explanation will be given more concretely. A seal material consisting of a thermosetting resin for forming a seal portion is printed on a fixed glass substrate. A movable glass substrate having a smaller thickness than the fixed glass substrate is put on the fixed glass substrate. Then, heat is applied to set the seal material while a pressing force is applied to the pair of glass substrates, thereby completing the touch panel.
When the seal material 33′ is pressed as shown in FIG. 42 during the production process of this touch panel, the width of the seal material 33′ expands to the outside and the inside of the touch panel, and an opposing gap between the pair of glass substrates 31a and 32a becomes small due to the pressing force and the surface tension of the resin of the seal material 33′. When the seal material 33′ is thermally set thereafter, the viscosity of the resin expands with the rise of the temperature in the setting process, so that the width of the seal material 33′ becomes even greater and the resin is set while the opposing gap becomes small.
As a result, the narrowness of the opposing gap between the pair of glass substrates 31a and 32a created by pressing the glass substrates 31a and 32a in turn invites the problem that so-called “Newton's rings” develop in the seal portion obtained after setting, as shown in FIG. 43. The occurrence of the Newton's rings is particularly remarkable at corner portions 33b of the seal portion 33. Symbol N represents the Newton's ring occurrence area.
To eliminate the Newton's rings, it has been attempted in the past to seal a gas into the space between the pair of glass substrates so as to raise the internal pressure of this space, to expand the movable glass substrate and thus to expand the gap. However, even when the gas is charged into the space between the pair of glass substrates and to raise the internal pressure of the space, the Newton's rings cannot be eliminated.
The inventor of the invention has specifically examined the occurrence position of the Newton's rings and has found that the Newton's rings occur particularly at the corners of the seal portions corresponding to the corner portions at the end portions of the glass substrates.
As to why the occurrence of the Newton's rings cannot be eliminated even when the gas is sealed, the inventors have found the following fact. Because the gas is charged after the seal portion is set, the seal portion is rigid during the expansion process of the glass substrate caused by charging of the gas. Therefore, the seal portion fails to follow the expansion of the glass substrate. When charging of the gas is stopped, the operation of returning the glass substrate to the original state acts on the glass substrate with the result that the gap between the pair of glass substrates is not expanded and the Newton's rings occur.
In view of the observation given above, this invention aims at providing a touch panel capable of preventing the occurrence of the Newton's rings and a production method thereof.